Edit: added another level 1 assembler not shown in this screen shot.
When 0.15 changed the recipe for Science Pack 3 I decided to design an early game build to take ingredients from belts off a bus, and produce 1 each of science pack 1,2 and 3 per second. I wanted to use the cheapest components necessary. i.e. Assembler 1 for items that have up to 2 ingredients, fast inserters only when absolutely necessary.